What is the g-value?

In modern architecture, every detail counts when it comes to energy efficiency, indoor comfort and sustainability. One of the key performance indicators in building design is the so-called g-value, also known as the total solar energy transmittance. It determines how much solar energy passes through a glazing system into the interior — and thus influences the thermal balance between light, heat and comfort.

But what exactly does this value mean? Why does it play such a central role for architects, planners and investors in commercial and institutional buildings? And how can innovative glazing solutions — such as the solar control systems from ISOLAR SOLARLUX® — achieve optimal g-values for any building situation?

The g-value is part of a group of physical parameters that describe the performance of a glazing system. Together with the U-value (thermal transmittance), it provides a complete picture of the thermal behaviour of the building envelope:

Important: While the U-value reduces energy losses, the g-value determines how much solar energy is blocked in summer and utilised in winter. The challenge lies in balancing both values optimally.

An important quality indicator is selectivity (S). It describes the ratio between light transmission (Lᵀ) and solar energy transmittance (g):

S = Lᵀ / g

The higher the value, the more daylight enters while heat gain is reduced. A selectivity value above 1.5 is considered very good — ISOLAR SOLARLUX® glazing reaches values up to 1.9, meaning excellent daylight utilisation with effective heat protection.

For planners, this provides greater design flexibility, as high transparency is possible without overheating.

Factors influencing the g-value

Several parameters affect how the g-value behaves in practice:

• Coating technology: Modern low-E and solar control coatings reflect infrared radiation while allowing visible light to pass through.

• Colour & reflection: Neutral glass avoids colour distortion, while slightly tinted variants can create targeted design accents.

• Internal systems such as SOLARLUX® variodirect: Venetian blinds inside the insulating glass unit enable variable control of the g-value — from open (high light transmission) to closed (minimal heat absorption).

These factors show: the g-value is not a fixed material property, but a planning parameter that can be adapted to usage, orientation and comfort requirements.

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A well-chosen g-value is a key contribution to sustainable building design. It reduces the need for air conditioning, improves thermal comfort and simultaneously ensures high light quality.

ISOLAR SOLARLUX® glazing combines high energy efficiency, thermal comfort, and architectural design freedom in a single system.

• low Ug-value for minimised heat loss,
• graduated g-values for different façade orientations,
• high colour neutrality and light transmission,
• combinable with functions such as sound insulation, safety glass or RF transparency.

This combination of energy efficiency and design freedom makes modern solar control glazing a central component of sustainable architecture.

The g-value as a planning tool for holistic architecture

The g-value is more than a performance figure — it is a tool for energy-optimised building design. Those who understand it and apply it correctly create spaces that adapt to the climate, rather than the other way around.

ISOLAR SOLARLUX® systems demonstrate that technological innovation and architectural aesthetics are not opposites:
They enable designers to combine high transparency, efficient solar control and minimal energy losses — and thus create sustainable buildings for the future.